Gemlik is a town and district in the Bursa Province in Turkey on the southern gulf of Armutlu Peninsula on the coast of the Sea of Marmara. It is located approximately 29 km (18 mi) from Bursa, not far from Istanbul. Gemlik was called Kios until 1922, when its Greek inhabitants, who composed around 80% of the population, left Asia Minor as a result of the population exchange between Turkey and Greece]. As of 2009, the total population of Gemlik is 100,000.

Gemlik harbor is one of the most important harbors in Turkey. The remains of the ancient Greek town of Kios can be found east of Gemlik. The city is renowned for olives and olive oils. Marmara Birlik, a cooperative of olive farmers and producers, it is ranked among the top olive producers in the world.

Gemlik is bordered on three sides by mountains, with the Samanli Mountains lying to the west. To its west lies the Sea of Marmara. There is a large plain spanning much of the east end of Gemlik to the coast. Karsak Stream, which originates at İznik Lake, divides this plain into two parts.

The coast of Gemokk is straight, except around Kayikhane.

The gulf of Gemlik, often called Incir port or Kilyos (in reference to the ancient name of the town), is 35 km from east to west, 10–15 km from north to south, and 100-150 meters deep. Tuzla and Kapakli lie at either end of the gulf. For many years, this gulf had been used for docking ships, but now it serves as a tourist attraction.

The largest river in the area is Karsak, near İznik Lake. Another river, Koca stream, is near Engürücük.

As of 1920, Gemlik was estimated to have a population of 5,000. At that time, it had a naval yard that was no longer in use.[3] On September 11, 1922, Gemlik's remaining Greek citizens were forced to flee to Greece due to pressure by Turkey to abandon the area.